10.2.1.1 Design Requirements

• VA = 3V
• VIO = 3V
• 3-Wire RTD using 2 current sources

10.2.1.2 Detailed Design Procedure

Figure 78 shows the first topology for a 3-wire resistive temperature detector (RTD) application. Topology 1 uses two excitation current sources, IB1 and IB2, to create a differential voltage across VIN0 and VIN1. As a result of using both IB1 and IB2, only one channel (VIN0-VIN1) needs to be measured. As shown in Equation 14, the equation for this channel is IB1 x (RTD – RCOMP) assuming that RLINE1 = RLINE2. Equation 14 is the VIN equation for Topology 1.

Equation 14.

The PT-100 changes linearly from 100Ω at 0°C to 146.07Ω at 120°C. If desired, choose a suitable compensating resistor (RCOMP) so that VIN can be virtually 0V at any desirable temperature. For example, if RCOMP = 100Ω, then at 0°C, VIN = 0V and thus a higher gain can be used.

The advantage of this circuit is its ratiometric configuration, where VREF = (IB1 + IB2) x (RREF). Equation 15 shows that a ratiometric configuration eliminates IB1 and IB2 from the output equation, thus increasing the overall performance. Equation 15 is for ADC_DOUT showing IB1 and IB2 eimination.

Equation 15.

10.2.1.3 Application Curve

Figure 79. PT-100 RTD Resistance from –200°C to 850°C

10.2.2.1 Design Requirements

• VA = 3V
• VIO = 3V
• 3-Wire RTD using 1 current source

10.2.2.2 Detailed Design Procedure

Figure 80 shows the second topology for a 3-wire RTD application. Topology 2 shows the same connection as topology 1, but without IB2. Although this topology eliminates a current source, it requires two channel measurements as shown in Equation 4.

Equation 16.

Equation 4 — VIN Equation for Topology 2

10.2.2.3 Application Curve

Figure 81. PT-100 RTD Resistance from –200°C to 850°C

10.2.3.1 Design Requirements

• VA = 5V
• VIO = 2.7V
• Thermocouple with Cold Junction Compensation

10.2.3.2 Detailed Design Procedure

The LMP90xxx is also ideal for thermocouple temperature applications. Thermocouples have several advantages that make them popular in many industrial and medical applications. Compare to RTDs, thermistors, and IC sensors, thermocouples are the most rugged, least expensive, and can operate over the largest temperature range.

A thermocouple is a sensor whose junction generates a differential voltage, VIN, that is relative to the temperature difference (Thot – Tcold). Thot is also known as the measuring junction or “hot” junction, which is placed at the measured environment. Tcold is also known as the reference or “cold” junction, which is placed at the measuring system environment.

Because a thermocouple can only measure a temperature difference, it does not have the ability to measure absolute temperature. To determine the absolute temperature of the measured environment (Thot), a technique known as cold junction compensation (CJC) must be used.

In a CJC technique, the “cold” junction temperature, Tcold, is sensed by using an IC temperature sensor, such as the LM94022. The temperature sensor should be placed within close proximity of the reference junction and should have an isothermal connection to the board to minimize any potential temperature gradients.

Once Tcold is obtained, use a standard thermocouple look-up-table to find its equivalent voltage. Next, measure the differential thermocouple voltage and add the equivalent cold junction voltage. Lastly, convert the resulting voltage to temperature using a standard thermocouple look-up-table.

For example, assume Tcold = 20°C. The equivalent voltage from a type K thermocouple look-up-table is 0.798 mV. Next, add the measured differential thermocouple voltage to the Tcold equivalent voltage. For example, if the thermocouple voltage is 4.096 mV, the total would be 0.798 mV + 4.096 mV = 4.894 mV. Referring to the type K thermocouple table gives a temperature of 119.37°C for 4.894 mV.